The invention concerns a crane, in particular a vehicle crane, comprising a first crane arm and at least one second crane arm which are connected together by way of a joint and which are pivotable relative to each other by way of a drive having a piston-cylinder unit, in each case starting from one of the two end positions in which the crane arms are in a substantially mutually parallel relationship until reaching a dead point when the longitudinal axis of the piston-cylinder unit is substantially on a line with the joint.
A conventional method of overcoming the dead point position of a piston-cylinder unit relative to the joint is that in which the crane arms are pressed with their load pick-up means against the ground beneath the crane arms. The torque which occurs in that case in relation to the joint moves the piston-cylinder unit beyond the dead point and the crane can be folded together. That method is extremely inexpensive as no additional material expenditure is necessary. The disadvantage of that method is that the operation of folding the crane together is relatively time-consuming and takes up a great deal of space as the crane arms—strictly speaking the load pick-up means—must make ground contact while the two crane arms are opened out. Often such cranes are designed so that the drive has not just one piston-cylinder unit but, for example, two piston-cylinder units which are disposed in parallel, and which acting symmetrically in the same direction move the two crane arms relative to each other. In that case, the dead point of the two piston-cylinder units occurs at the same position of the joint, and the movement for going beyond the dead point position has to be effected in the same fashion as just described.
DE 69 16 283 discloses a rotary joint arrangement for folding booms, having an intermediate member between the two crane arms and two stroke cylinders mounted to the crane arms and the intermediate member. The overall pivotal angle is increased by one stroke cylinder implementing a pivotal range of about 160 degrees and the second stroke cylinder implementing a further additional pivotal range of about 120 degrees. That, therefore, avoids having to overcome the dead point of a stroke cylinder as neither of the two stroke cylinders has to be extended as far as its dead point. In that way, the working ranges are sharingly allocated to the two stroke cylinders. The disadvantage of this method is that it involves an increased expenditure on material and thus increased costs as an intermediate member and two powerful stroke cylinders are necessary to achieve a pivotal range for the two crane arms, which is greater than 180 degrees.